It is the primary goal of the proposed investigations to provide a more substantial basis for understanding protein stability via direct measurement of thermodynamic parameters (particularly volume). Auxillary to this we propose to examine the validity of the two-state hypothesis, both on the basis of such measurements and through experimental examination of transitions for stable intermediate states. Experimental determination of protein volume will be effected over a range of pressure, temperature, and chemical potential employing optical interferometric techniques. Proposed investigations are planned to complement calorimetric measurements now becoming available from other laboratories. Experiments in which proteins in the transition region and at equilibrium are physically separated according to conformational state are proposed as a direct test for intermediate states.